The aim of the proposed program is to develop a series of high energy density long life implantable power sources with power levels ranging from microwatts to several watts. Health applications range from power for cardiac pacemakers to heart assist pumps. The core of the proposed device is a radioisotope powered indium phosphide voltaic cell which can achieve energy densities over ten thousand times greater than that of the best chemical batteries. In past attempts to develop radioisotope powered voltaic cells (beta-voltaic cells) the power attained was severely limited because of radiation damage to the silicon semiconductor material used in them. The recently discovered radiation damage resistance and annealing properties of indium phosphide remove this fundamental limitation and permit consideration of high energy beta and alpha emitters to achieve high power and long life. The possibility of using alpha emitters can also result in a cell with almost no detectable external radiation. The Phase I research will provide the key experimental and theoretical data required to translate the new technology into prototype power cells for application tests. This technology could be applied to all cases where long life without replacement or recharging is required.